The field of the invention is nuclear magnetic resonance imaging methods and systems. More particularly, the present invention relates to the tuning of a resonant coil which is excited to create a radio frequency magnetic field.
Any atomic nucleus which possesses a magnetic moment attempts to align itself with the direction of the magnetic field in which it is located. In doing so, however, the nucleus precesses around this direction at a characteristic angular frequency (the Larmor frequency) which is dependent on the strength of the magnetic field and on the properties of the specific nuclear species (the magnetogyric constant q of the nucleus). Nuclei which exhibit this phenomena are referred to herein as "spins".
When a substance such as human tissue is subjected to a uniform static magnetic field (polarizing field B.sub.z), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. A net magnetic moment M.sub.z is produced in the direction of the polarizing field, but the randomly oriented magnetic components in the perpendicular, or transverse, plane (x-y plane) cancel one another. If, however, the substance, or tissue, also is subjected to a radio frequency excitation field (B.sub.1) which is in the x-y plane and which is at the Larmor frequency, the net aligned moment, M.sub.1, may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M.sub.z, which is rotating, or spinning, in the x-y plane at the Larmor frequency. The degree to which the net magnetic moment M.sub.z is tipped and, hence, the magnitude of the net transverse magnetic moment M.sub.1, depends primarily on the length of time and magnitude of the applied RF excitation field B.sub.1.
The practical value of this phenomenon resides in the signal which is emitted by the excited spins after the RF excitation field B.sub.1 is terminated. In simple systems the excited spins induce an oscillating sine wave signal in a receiving coil. The frequency of this signal is the Larmor frequency, and its initial amplitude, A.sub.0, is determined by the magnitude of the transverse magnetic moment M.sub.1. The amplitude, A, of the emission signal decays in an exponential fashion with time, t, according to the equation: EQU A=A.sub.0 e.sup.t/T*.sub.2
The decay constant 1/T*.sub.2 depends on the homogeneity of the magnetic field and on T.sub.2, which is referred to as the "spin-spin relaxation" constant, or the "transverse relaxation" constant. The T.sub.2 constant is inversely proportional to the exponential rate at which the aligned precession of the spins would dephase after removal of the RF excitation field B.sub.1 in a perfectly homogeneous field. The signal emitted by the excited nuclei have particular application for medical imaging of the anatomical features of live human patients.
NMR imaging systems generate the excitation magnetic field B.sub.1 using a body coil which is capable of handling large amounts of RF power to provide a homogeneous RF magnetic field throughout a large region. An example of a body coil is shown in U.S. Pat. No. 4,692,705 and is commonly referred to as a "cage coil" in that it is formed by two conductive end loops spaced apart along a central axis and interconnected by a number of axial conductive segments creating the appearance of a cage. Each of the conductive segments is provided with at least one reactive shunt such as a capacitor and the end loops also have a plurality of serially connected reactive shunts. This forms a tuned coil designed to resonate at the Larmor frequency.
The body coil is typically excited at two of its conductive segments spaced 90 degrees apart around the coil by two RF signals which are in quadrature. As a result of this excitation, the end loops of the coil carry a sinusoid distribution of current around them, thereby creating two orthogonal resonant modes within the coil. One mode carries current proportional to the sine of the angle around the loop, while the other mode carries current proportional to the cosine of that angle as given by: EQU V.sub.1 =cos .omega..sub.1 t and V.sub.2 =sin .omega..sub.2 t
where .omega. is the frequency of the signal traveling around the coil. In a properly tuned NMR body coil, both of these modes will resonant at the Larmor frequency, (i.e. .omega..sub.1 =.omega..sub.2).
With a multi-element cage coil, the resonant frequency is primarily determined by the distributed inductances and discrete capacitances built into the resonance structure of the body coil. In order for the two modes to resonate at the same frequency, the reactance of each section of the coil must be uniform. Unfortunately, realistic manufacturing tolerances of the coil and its various reactive components make it virtually impossible to have a uniform reactance in all sections of the coil. This variation in reactance from section to section around the coil produces a separation in the resonant frequencies of the two modes, as well as a deviation from the desired Larmor frequency. Therefore, body coils have had to include some form of tuning mechanism in order to adjust the reactance of the coil sections so that both modes resonated at the Larmor frequency. This has been previously accomplished by introducing adjustable tuning elements in the end loops and conductive segments of the coil which cancel the effects of the tolerance variation in the coil elements. In a production environment, it is difficult and laborious to tune a coil with these elements since it is impossible to predict the angular direction of the two resonant modes and therefore, where to place tuning elements. This difficulty is compounded by the physical properties of the coil, which cause it to operate in the two orthogonal resonant modes where the frequency separation between the modes is the greatest. This tendency toward selecting the largest available frequency difference causes the coil to shift its modes into a new position whenever the frequency difference in the present modes is adjusted to zero. Thus, these adjustable tuning elements must be introduced into as many positions of the coil as practical in order to provide a combination of tuning elements in each direction where tuning is desired. The greater the number of tuning elements, the more complex the tuning task becomes.